Diesel engines are operated at higher than stoichiometric air to fuel mass ratios for improved fuel economy. Such lean-burning engines produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (NOx). The temperature of the exhaust from a warmed up diesel engine is typically in the range of 200° to 400° C. and has a representative composition, by volume, of about 10–17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NOx and the balance nitrogen and water.
These NOx gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content in the hot exhaust stream. It is, thus, an object of the present invention to provide an improved method of reducing NOx in such gas mixtures. It is a more specific object of the present invention to provide a method of modifying diesel exhaust with reformed diesel fuel before the exhaust is treated with a zeolite type NOx reduction catalyst.